1. Field of the Invention
This invention relates to a coating composition which provides improved resistance to environmental attack of metallic substrates and markedly higher "throw" (as hereinafter defined), and which can be applied by any conventional coating technique. The coatings deposited by the composition of the invention may be electrically non-conductive but are nevertheless highly cathodic when applied over a ferrous metal substrate, and can be recoated with any topcoat at any time.
2. Description of the Prior Art
Zinc-rich primers having an inorganic binder or vehicle have been in use for about 40 years. Typically such compositions contain about 80% to about 95% by weight powdered zinc, and alkyl silicate has been the inorganic binder of choice. Single-package primer compositions containing about 80% to about 90% by weight zinc and ethyl silicate binder have been in use for about the last 20 years.
A brochure published by AKZO N.V. (publication date unknown) discusses the development of zinc-rich primers, the preparation of ethyl silicate and its use and mechanism as a binder. This publication explains that ethyl silicate is derived from tetraethyl orthosilicate which is reacted with water in the presence of an organic solvent to produce liquid polysiloxane resins. When used in a zinc-rich primer, evaporation of the solvent after application of the coating results in transformation of the polysiloxane resin to amorphous silica, which becomes the bonding film in the cured coating. The amorphous silica reacts chemically with some of the zinc powder in the coating to form zinc silicate compounds. The silica will also react with the ferrous metal substrate, particularly if it is previously abraded or sandblasted, to form iron-(zinc)-silicate bonds. Silicate mineral extenders frequently used in zinc-rich primers also are believed to react chemically in such inorganic systems. The resulting bonding matrix allows a controlled galvanic current flow between the ferrous substrate and the zinc pigment (cathodic and anodic to one another, respectively), thus providing long term galvanic protection, including scratched or abraded bare areas of substrate due to adjacent zinc metal.
U.S. Pat. No. 4,417,007, issued Nov. 22, 1983, to Salensky et al., discloses a zinc-rich paint formulation containing manganomanganic oxide as a color pigment, in which the binder may be any one of (1) epoxy resins, (2) that derived by reaction from diglycidyl ether of bisphenol A and vegetable oil fatty acids, (3) that derived from bisphenol A and epichlorohydrin, or (4) alkyl silicate. From about 43% to 90% by weight zinc dust, and from about 3% to 38% manganomanganic oxide are present, along with from about 4% to 25% by weight epoxy resin binder, 0 to about 35% by weight pigment extenders and binders, 0 to about 5% by weight of a pigment suspension agent and balance solvent, in a claimed embodiment.
U.S. Pat. No. 4,891,394, issued January, 1990 to R. R. Savin, discloses a coating composition comprising about 10% to about 25% by weight of a film-forming polymer which may be an epoxy resin, a vinyl chloride resin copolymerized with polyisocyanates, or a vinyl chloride resin copolymerized with melamines; about 30% to about 60% by weight particulate metallic zinc; a crystalline silica having an oil absorption value of less than 20 measured by ASTM Test D281-84, the volumetric ratio of such silica to zinc ranging from about 0.7:1 to about 1.25:1; about 2% to about 3% by weight of a pyrogenic amorphous silica having an average particle size less than about 0.012 micron (for control of rheological characteristics); and at least one solvent for the film-forming polymer.
French Patent 8611238 (Publication No. 2,602,239), published February, 1988, in the name of R. R. Savin, discloses a two part coating composition containing up to 70% by weight of a powdered metal (based on the total weight after admixture); about 2% to 30% by weight of a film-forming polymer (as an organic binder); about 2% to about 30% of a hardener for the polymer; at least 1.8% and up to 30% of an agent for control of rheological characteristics; and up to 30% organic solvents. A preferred polymer is an epoxy resin having an average molecular weight of 350 to 3800. The agent for control of rheological characteristics includes at least one pyrogenic silica and optionally at least one natural or transformation silica having an oil absorption value preferably not greater than 90 and more preferably not greater than 40.
U.S. Pat. No. 5,098,938, issued March, 1992 to R. R. Savin, discloses a coating composition similar to that of the above-mentioned U.S. Pat. No. 4,891,394, wherein an epoxy resin film-forming binder is used, and wherein at least four different size grades of pyrogenic amorphous silicas are present within specified proportions and average particle sizes, together with a crystalline silica having an oil absorption value of less than 20 measured by ASTM Test D281-84.
Conventional zinc-rich primers having an inorganic binder exhibit the following disadvantages:
Sedimentation or settling requires continuous agitation before and during application.
Recoatability of inorganic binder primers is poor.
Sandblasting or abrading of the metallic substrate is needed for best results.
Application by brushing or flowcoating is not recommended for inorganic binder primers.
The density of the composition is high due to the high metallic zinc content.
Prior art zinc-rich primers having organic binder suffer from the following disadvantages:
Prolonged exposure to acetone, methylene chloride and/or methylethylketone will cause eventual deterioration of the coating.
Heat resistance is relatively low, i.e., loss of properties when subjected to temperatures above 175.degree. C. (350.degree. F.) and destruction of the coating when subjected to temperatures above about 260.degree. C. (500.degree. F.).
The "throw" properties are poor as measured by the so-called bullet test, which determines the distance and/or area of cathodic action around a bare area of substrate.
While progressive improvements in general resistance to environmental attack of metallic substrates are disclosed in the prior art acknowledged above, there is nevertheless a genuine need for a one-package zinc-rich coating composition which avoids all the disadvantages pointed out above inherent in both inorganic and organic binder zinc-rich compositions.